Magnetic resonance imaging (MRI) technology has advantages such as intensive information, multi-directional imaging and high resolution. Therefore, since its first application in 1980, MRI technology has developed rapidly and has been applied widely as an important means for clinical disease diagnosis.
As one of the important components of a MRI imager, a gradient amplifier is used to drive gradient coils in the MRI imager to generate gradient magnetic field to provide positional information. In order to reduce the imaging time and ensure the image quality, the MRI system requires a high-resolution, high-precision, stable strong magnetic field to be established in the coil in a short time. Therefore, the gradient amplifier is required to have the ability to output high voltage as well as high precision and large current. The gradient coil in the nuclear magnetic resonance instrument is generally a hollow inductor with a length of 2 m and a diameter of 90 cm. In order to shorten the time for establishing the magnetic field, when the magnetic field is established, the current in the coil is required to have a high slew rate, so that the current in the inductor can quickly reach a preset value; and after the magnetic field is established, it is necessary to maintain the required large current accurately and stably in the coil, and the slew rate of the current at this time is low. Taking a typical current waveform (i.e. a trapezoidal wave) on a gradient coil, shown in FIG. 1 as an example, the process of establishing the magnetic field corresponds to a rising edge stage and a falling edge stage in the trapezoidal wave. The slew rate of the current is extremely high in those stages. After the magnetic field is established, the current is maintained at the preset value precisely and stably, and at this time, the current slew rate is extremely low. Therefore, the gradient amplifier used to drive the gradient coil is required to have the ability to output high voltage to obtain a higher slew rate of current, and to maintain the output high-precision and large current at a low output voltage too.
The available gradient amplifiers in the market today generally employ full-bridge converter technology. Due to the characteristics of silicon-based devices, most of the gradient amplifiers apply high-low voltage bridges cascade structure, whose disadvantages include uneven thermal distribution problem, low device utilization, poor reliability, and complicated system expansion.